We have isolated and characterized recombinants and nonconditionally defective mutants of Rous sarcoma virus. Four mutants have been analyzed in detail. (1) SE 21Q1b has a 150 bp deletion at the 5' end of the provirus genome and a defect in RNA packaging; (2) SE 52d contains a defective polymerase gene which encodes an aberrant gag-pal protein. The mutation may involve substitution of quail cellular information for part of pol; (3) SE 521 synthesizes a glycoprotein precursor which is not cleaved into mature gp85/gp37; (4) SE 33 synthesize a small gag-related protein with p19 determinants. Further characterization of these and other mutants is in progress.